Conventionally, a free piston engine is known as an engine that needs no crank, no connecting rod, and no piston pin bearing and that can be compact. Further, in Patent document 1, a power generation system uses a free piston engine as its power source. Here, the free piston engine has a magnet portion disposed in a center of an axial direction, and a shaft that has pistons at both ends. The pistons in the both ends are contained within both cylinders, respectively. The both cylinders alternately undergo explosions of gases including gasoline, so that the magnet portion is axially reciprocated.
A different type of a free piston engine is also known in Patent document 2. Here, two opposing pistons are contained within a single cylinder. Pressure chambers are provided rearwards of the respective pistons. Gases are exploded within a space between the opposing pistons, so that axial driving force is generated in a direction. Gases filled in each of the pressure chambers rearwards of the piston are then compressed, so that the piston is pushed back in the other direction. The piston is thereby returned towards the combustion chamber to reciprocate. Further, in Patent document 3, a free piston engine with external-combustion of the Starling cycle is known as a free piston engine that uses a coil spring for pushing back the piston.
Patent document 1: JP2001-241302A
Patent document 2: U.S. Pat. No. 3,370,576
Patent document 3: U.S. Pat. No. 4,450,685
To maintain the reciprocating motion, pushing back of the piston towards the combustion chamber becomes necessary after a combustion stroke within the combustion chamber. In Patent document 1, the combustion stroke takes place alternately in the both-end combustion chambers, so that the combustion stroke in the one-end combustion chamber pushes back the piston towards the other-end combustion chamber. However, in a two-cylinder free piston engine constituted by the two combustion chambers, periodic vibrations due to the explosions cannot be easily decreased. Therefore, the number of cylinders needs to be increased so that the generated vibrations can be offset. This results in a large-sized free piston engine due to the multiple cylinders, which cancels an advantage of the free piston engine that is suitable for reducing the size.
In Patent document 2, two pistons face each other within a single cylinder, so that the two pistons reciprocate in the phases opposite to each other. The generated vibrations can be thereby decreased. However, to reciprocate the two pistons in the opposite phases, the shifted piston in the combustion stroke must be returned back by a pushing-back method. Suppose that the piston is returned back by the gases filled within the pressure chamber. Here, each time the gases are compressed and expanded, the kinetic energy of the pistons is converted into the thermal energy that is released. As a result, the energy generated from the explosion of the gases within the combustion chamber cannot be effectively extracted. Further, the gases generate non-linear reaction force to a displacement amount of the piston, and further variable properties to temperatures or pressures. Moreover, this requires a method for filling the gases within the pressure chamber, and a valve for controlling gas pressure within the pressure chamber. This eventually entails difficulty in control of the free piston engine, complication in structure, and a large size.
In the above Patent documents 1, 2, 3, the movements of the pistons are performed by sliding with the cylinders. The contact surfaces of the pistons and cylinders easily undergo friction, resulting in decrease in durability. Further, the circumferential rotations of the pistons are not easily controlled. Therefore, when the free piston engine is applied to the motive engine for a generator, a shaft connected to a piston needs being provided with a magnet portion that is disposed in a cylindrical form uniformly around the circumference of the shaft. Suppose that the magnet portion is disposed in the cylindrical form and that electric power or driving force is generated from the generator. The surface area where the magnetic field varies becomes small, which results in decrease in efficiency. When the surface area where the magnetic field varies is expanded, the diameter of the magnet portion must be enlarged. This entails increase in weight and size in the free piston engine.